This invention concerns a circular tool for cutting rolls of paper and similar as set forth in the main claim.
The tool according to the invention is used in cutting machines employed in the paper industry to cut transversely the reel of paper so as to obtain rolls of defined length and diameter.
A particular, though not exclusive, application of the invention is to use the tool for the cyclical cutting of rolls of toilet paper or absorbent paper for domestic use.
The state of the art includes cutting machines used in the paper industry to subdivide the reels of paper into several rolls of a desired length.
These machines are usually equipped with one or more rotary tools, or disks, which have a sharp circumference and are keyed onto a supporting shaft equipped with gripping flanges.
These machines are suitable to transmit to the supporting shafts of the disks both a rotation movement around the relative axis, and also a revolving movement in order to sequentially cut the reels of paper which are fed forwards linearly in correspondence with the revolving circumference of the disks.
Cutting disks known in the state of the art have a transverse section which is symmetrical with respect to a median longitudinal plane and is defined by a first brief inner segment, astride the axis of rotation of the tool, substantially plane and suitable to cooperate with the gripping flanges, and by a second outer segment, slightly wedge-shaped.
The top of said second outer segment is sharpened on both sides by means of symmetrical bevelling, or chamfering, of an extremely limited angle, in the order of about 8xc2x0 or a little more.
An example of such conventional cutting tools is shown in DE-A-39 06 026, which discloses the features of the preamble of claim 1.
These disks usually have a diameter of around 600 mm, or more, and a thickness of around 5 mm: they are required to perform up to 800 strikes, or cuts, a minute, which compromises the sharpness and therefore the productivity of the tool.
Therefore, such blades need frequent sharpening, and this is usually done in line by means of pairs of grinding elements arranged in xe2x80x9cVxe2x80x9ds and suitable to restore the sharpness to the cutting edge of the disk.
The grinding elements do not have their own drive and are taken into rotation under pressure from the rotation movement of the tools themselves.
However, re-sharpening the cutting edge in this way is not very efficient, inasmuch as it is very difficult to center the grinding elements and in the long term it may make the disk slightly elliptic, which compromises the accuracy of the cutting action.
Moreover, the re-sharpening carried out by these grinding elements, also because of the great pressure exerted thereby, makes the edge of the cutting blade very jagged, often causes imperfections in the cut section of the roll of paper, and leads to a rapid deterioration of the tool.
The jaggedness of the edge compromises the cutting efficiency of the tool, especially when cutting rolls of padded paper or such like: such paper is very delicate to cut and requires extremely sharp tools to obtain high quality results.
Moreover, since the re-sharpening process is usually repeated every 50 strikes, or cuts, according to the type of paper to be cut, it entails a premature wear of the disks, a need to replace them rapidly and therefore an increase in the management costs of the cutting machines.
One shortcoming which businessmen in this field especially complain of is the poor quality of the geometry of the cut, owing in particular to the prevalently wedge-shaped conformation of the disk.
This wedge-shaped conformation is required by the large diameter and thickness of the disks, and causes considerable pressure on the cut edge of the roll; this often generates a cut which is not at right angles and deforms the edge of the roll.
A further disadvantage is that conventional tools employed in the field of cutting rolls of paper, such as for example the tool described in DE""026 or that described in DE-C-196 16 678, are lined with protective material on both sides, which entails the need to provide a new lining after every sharpening operation because the lining is removed precisely during the sharpening operation.
EP-A-628.379 describes a cutting tool for use with food products, with a transverse section substantially symmetrical with respect to a median axis and with a cutting profile lined on one side only by protective material.
This tool is not suitable to cut products such as rolls of paper, which require a very sharp edge.
Moreover, the symmetry of the section gives it an extremely short life, since after only a few sharpening operations the cutting profile is removed to such a point that the tool is unusable and has to be replaced.
The present applicants have devised and embodied this invention to overcome these disadvantages.
The invention is set forth and characterised in the main claim, while the dependent claims describe other characteristics of the main embodiment.
The purpose of the invention is to provide a circular tool, or disk, to cut rolls of paper and suitable to make cuts with an extremely precise profile, perfectly orthogonal to the longitudinal axis of the roll and without deforming the cut edge of the roll, particularly rolls of padded paper or similar.
Another purpose of the invention is to drastically increase the working life of the tool, and thus reduce the management costs of the cutting machines and increase their productivity.
A further purpose is to obtain a tool which will be reliable even mounted on a machine, which will require much less frequent sharpening operations compared with conventional tools and which will keep its cutting characteristics intact even after a plurality of sharpening operations.
The circular tool according to the invention has a transverse section defined by two lateral surfaces which are not wedge-shaped, but are parallel to one another.
In this way, during the cutting step, any lateral pressure on the edge of the roll is eliminated and therefore the resulting cut is perfectly orthogonal with respect to the longitudinal axis of the roll. Moreover, the edge of the roll is not deformed by the cutting action.
The outer cutting edge of the circular tool is defined by two bevels, or chamfers, present on both sides of the edge.
In the preferential embodiment of the invention, the chamfers have an inclination of between 6xc2x0 and 10xc2x0 with respect to the longitudinal median plane of the disk.
In an even more preferential embodiment, the chamfers have an inclination of an angle of around 8xc2x0 with respect to the longitudinal median plane of the disk.
According to one characteristic of the invention, only one of the two surfaces defined by the chamfers, that is to say, the one which is not directly concerned with cutting, is covered with a protective layer made of an extremely hard material such as, for example, a nitride.
In this way, the cutting edge is maintained much longer compared with tools known to the state of the art, since it is defined, on one side, by this layer of very hard material.
Moreover, this cover lasts much longer in that the chamfered side which is subjected to sharpening is the one which is not covered, and therefore the sharpening operation does not remove the covering layer.
According to another characteristic of the invention, the chamfers are asymmetrical with respect to the longitudinal median plane of the tool.
In a preferential embodiment, a first chamfer, that is to say, the one not lined with protective material, is made on about one third of the thickness of the tool, and the other chamfer, that is to say, the one which is lined, is made on about the remaining two thirds of the thickness.
This asymmetry, wherein the narrowest part of the profile comprises, at least initially, the side of the tool which is used for cutting, prolongs the working life of said tool.
In fact, the sharpening operations made on the cutting side, by progressively removing material from the outer edge of the tool, progressively displace the asymmetry until the cutting profile is first made symmetrical and then asymmetrical in a specular manner with respect to the initial profile.
This configuration, together with the less frequent sharpening operations, means that the tool can be used for a much larger number of cutting cycles compared with tools known to the state of the art.
In a first embodiment of the invention, the tool is made of high speed steels while, according to a variant, it is made of powder steel.
The tool according to the invention, in its preferential embodiment, has an outer diameter of between 500 and 550 mm.
In an even more preferential embodiment, the tool has an outer diameter of around 530 mm.
The smaller diameter compared with tools such as are known to the state of the art makes possible to achieve tools with parallel walls, not tapered, and also to use a much reduced thickness, of between 2 and 4 mm, advantageously in the range between 2.8 and 3.2 mm, without any danger of the tool bending during the cutting action.
The fact that the lateral surfaces of the tool are parallel, and the reduced thickness of the tool, make possible to achieve very accurate cuts, perfectly orthogonal to the longitudinal axis of the roll.
The reduced thickness also allows to reduce the friction and pressure between the tool and the paper, which gives the advantage of cleaner cuts and less effort required for the drive means of the cutting machine.
According to a variant of the invention, used particularly with tools of limited diameter and for cutting particular types of paper, at least one of the lateral surfaces of the tool has a concave segment, which allows to further reduce friction and to prevent deviations in the cutting and jagged edges in the paper; this is thanks to the reduced pressure which the tool exerts on the cut edge of the roll in correspondence with the said concave segment.